Gilbert Mantel Clock - Hour Chime Out of Sync

by Bruce Altobelli
(Etna, NH)

I have an old Gilbert 8 day mantel clock with a bim bam chime. The clock itself keeps perfect time. The problem is that the hourly chimes get out of sync with the clock. The chimes get ahead of the actual time or in other words when the clock says 8:00 the chimes will ring 9. If not corrected, the chimes will continue to advance an hour almost randomly.
On occassion I can hear a thunking sound as if one of the mainsprings suddenly and partially unwound.